Caption: Animation of the structure of the 5g z3 atomic orbital. Orbitals contain an atom's electrons, around the central atomic nucleus (not seen). Each orbital can hold two electrons. The 5g orbital is the first g orbital, after s, p, d and f orbitals, and would only be filled in the ground state of element 121, which has never been synthesised. Orbitals further from the nucleus are at a higher energy, and orbitals are filled from the lowest energy (level one) first. An atomic orbital is a mathematical model of the location of an electron around an atomic nucleus. Electrons do not orbit the nucleus as might be imagined, but instead exist as a standing wave around it. The phases of the wave are arbitrarily coloured red (negative) and blue (positive). S orbitals are spherical, and comprise concentric shells of alternating positive and negative wave function phases.